Non-contact magnetic sensor system

ABSTRACT

A non-contact sensor system according to one embodiment comprises a substrate having at least one sensor element, the at least one sensor element being directed towards at least one data track on a medium positioned opposite the at least one sensor element, wherein the substrate and the medium each carry at least one magnetic track, wherein orientations of magnetizations of the magnetic tracks are such that the substrate experiences a force away from the medium. Other embodiments are also presented.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/685,078 filed Mar. 12, 2007 now U.S. Pat. No. 7,355,399, which is acontinuation of U.S. patent application Ser. No. 10/982,221 filed Nov.5, 2004, now U.S. Pat. No. 7,208,948, which are both incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to a high-resolution sensor system, andmore particularly, this invention relates to a high-resolution sensorsystem that works in non-contact mode.

BACKGROUND OF THE INVENTION

Devices for quantative detection of linear and rotary movements areknown. Optical encoders are used to detect the rotation angle or,respectively, a length and a direction of a rotary movement or,respectively, linear movement of moving bodies. The essential componentsof such a device are the emitter system, a grid plate, normally a griddisk or a grid straight edge, and the detector system. The emittersystem normally contains a light emitting diode (LED). The light beamemitted from the light emitting diode or laser diode is modulated by thegrid plate. The grid plate is connected to a moving body and has aperiodic opening pattern. The detector system detects the transmittersignal (modulated by the grid plate) from the laser diode and, at theoutput, supplies information relating to the light beam and thedirection of movement.

High-resolution magnetic encoders using Hall sensors are also known. Aswell, magnetic encoders (magneto-electric converters) which employ amagnetoresistance effect element made of a thin ferromagnetic film, havebeen commonly used in various fields due to their good durability in asurrounding atmosphere, wide operational temperature range and highresponse frequency. For example, magnetic encoders are used forcontrolling the rotational speed of a capstan motor in a video taperecorder or the like. Generally speaking, magnetic encoders are used forpositional or speed control in factory automation (FA) equipments, suchas servomotors, robots and the like, or in office automation (OA)equipments, such as computer-printers and copying machines. In recentyears, there has been an increasing demand for improving the accuracy ofsuch equipments. In general, the magnetic encoder includes a magneticrecorder and a magnetic sensor disposed in opposition to the magneticrecorder. The magnetic recorder comprises a non-magnetic substrate and arecording medium which is a permanent magnetic material coated on theperipheral or flat surface of the non-magnetic substrate. The recordingmedium is magnetized in a multipolar fashion at a magnetizing pitch λ toform at least one magnetic signal track.

A hard disk drive (HDD) is a digital data storage device that writes andreads data via magnetization changes of a magnetic storage disk alongconcentric information tracks. During operation of the HDD, the disk isrotated at speeds in the order of several thousandrevolutions-per-minute (RPM) while digital information is written to orread from its surface by one or more magnetic transducers. To perform anaccess request, the HDD first positions the sensor and/or write head,also referred to as a “read/write head”, at the center of the specifieddata track of the rotating disk.

During operation of the HDD, the read/write head generally rides abovethe disk surface on a cushion of air, caused by an “air bearing surface”that is created by the movement of the disk under the head. The distancebetween the read/write head and the disk surface while riding, orpartially riding, on the air cushion is referred to as the “flyingheight” of the head.

Further, the head is carried by a “slider” which is supported byhydrodynamic lift and sink forces. These lift forces are given by theinteraction of air streaming underneath the surface structure of theslider.

To build encoder applications with high resolution, it is important tominimize the gap between the sensor and the information track.

As the air bearing surface varies with the rotation speed, using airpressure as with the HDD applications is not possible if the variationof the relative movement is too high.

The known optical encoders are limited to a small temperature range dueto high sensitivity of the used sensor/encoder-optics to temperaturechanges. The resolution of these encoders is also very sensitive to dustand humidity of the environment.

Hall sensors are very sensitive to temperature changes and thus can alsonot be used in a wide temperature range as required in the field ofautomotive applications, industrial applications or the like.

What is therefore needed is a high-resolution magnetic encoder systemthat overcomes the disadvantages of the prior art systems.

SUMMARY OF THE INVENTION

A non-contact sensor system according to one embodiment comprises asubstrate having at least one sensor element, the at least one sensorelement being directed towards at least one data track on a mediumpositioned opposite the at least one sensor element, wherein thesubstrate and the medium each carry at least one magnetic track, whereinorientations of magnetizations of the magnetic tracks are such that thesubstrate experiences a force away from the medium. Other embodimentsare also presented.

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, which, when taken inconjunction with the drawings, illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and advantages of the presentinvention, as well as the preferred mode of use, reference should bemade to the following detailed description read in conjunction with theaccompanying drawings.

FIG. 1 schematically depicts an embodiment of the inventive system;

FIG. 2 is a cross sectional view schematically showing the arrangementof the magnets in the components of the inventive system;

FIG. 3 is a chart of force vs. the gap height in the inventive system;

FIGS. 4A and 4B depict a design of the levitation magnets of FIG. 1; and

FIG. 5 depicts a chart representing the forces for the design in FIG. 4.

BEST MODE FOR CARRYING OUT THE INVENTION

The following description is the best embodiment presently contemplatedfor carrying out the present invention. This description is made for thepurpose of illustrating the general principles of the present inventionand is not meant to limit the inventive concepts claimed herein.

The inventive magnetic sensor system can be used, for example, as aread/write encoder for the detection of a length, velocity, anddirection of a rotary movement or, respectively, linear movement ofmoving bodies. It can also be applied in all cases, where a smalldistance is required in order to measure properties of a material withhigh resolution, or it can be used as a tool or as a combined system,i.e., a tool and a sensor. If the present invention is used as a tool,the sensor (8 in FIG. 1) can be replaced by an actuator, e.g., anelectrical coil, to produce heat. Since the coil can be very small(having a diameter of about 1 μm or below) and the tool-surface distanceis very small as well (also below about 10 μm, preferably below about 1μm), the surface can be molded at dedicated locations.

In the following description, the application of the invention as amagnetic encoder will be described. In this embodiment, the sensorelement may be a GMR (Giant Magneto-Resistive) or a TMR (TunnelingMagneto-Resistive) sensor of the type standard in the HDD art. Thesensor has no contact with the magnetic media and the whole system maybe encapsulated. Also, it should be kept in mind that the sensordescribed below can be found in a standard read/write head of a typeknown in the art. Further, the sensor can be replaced with an opticalemitter/sensor or Hall sensor with appropriate opposing surfaces.

A preferred embodiment implements a magnetic system with a rotatingmagnetic substrate such as, e.g., a magnetic disk, and minimizes the gapbetween the sensor and a data or information track on the disk by meansof magnetic forces that are independent of any relative movement betweenthe sensor and the disk.

One especially advantageous configuration of the inventive non-contact,high-resolution magnetic sensor system is similar to current HDD slidertechnology using air pressure (air bearing principle) for establishing anearly constant distance between the magnetic sensor and the disk.However, the proposed solution differs from that technology in that thepressure of the air is replaced by the pressure/force of a magneticfield to control the distance between the disk surface and the slider(magnetic bearing principle).

Air pressure cannot be used if the relative movement between the sensorand the disk surface varies, as in present sensor applications where themagnetic disk together with the magnetic sensor are used as magneticencoder. Likewise, non hard-drive applications have an operation rangewhere the air pressure method is not applicable. On the other hand, amagnetic force between the slider and the disk surface is independent ofany relative movement between those components.

This invention replaces the air bearing effect with the force of amagnetic field to control the distance between the surface and theslider. The invention has the advantage that the magnetic force isindependent of any relative movement, and therefore, this method can beapplied to any surface independent of shape and movement between theslider and the surface in order to create a non-contact high-resolutionsystem.

As shown schematically in FIG. 1, a slider 2, having a substantiallyplanar surface 4 is mounted on a suspension 6 as a guiding element. Theslider carries a sensor element 8, e.g., a magnetic, electrical or forcesensor or an actuator, e.g., a coil to apply a magnetic field, a heatingelement or the like. The suspension 6, that can include a spring,pneumatic/hydraulic pressure element, a magnet or the like, biases theslider 2 against a surface 10 of a magnetic medium 12, such as, e.g., amagnetic disk. Mounted on or formed in the magnetic disk 12 is a datatrack 14 that is arranged opposite the sensor 8. The slider 2 and themagnetic disk 12 both include at least one magnetic levitation track 16,18, which can be either permanent or electromagnetic. The orientation ofthe magnetization of the levitation tracks 16, 18 are designed such thatthe slider 2 receives a vertical force (relative to the plane of thedisk 12) away from the magnetic disk 12. This force counteracts thebiasing of the suspension 6 to create a dedicated distance between theslider 2 and magnetic disk 12. The slider 2 is thus flying on adetracting (opposing) force created by the magnets 16, 18. The distancebetween the slider 2 and the magnetic disk 12 depends on the magneticforce and the suspension force. The magnetization also applies ahorizontal force to the slider if the slider is not on its track. Thisforce keeps the slider aligned with the proper track.

FIG. 2 is a cross sectional view schematically showing the arrangementof the magnets in the components of the inventive system. The slider 2,carrying the sensor element 8 has a slider magnet 20, the south pole 20Aand the north pole 20B of which are connected by yoke 22. The directionof the magnetic force is indicated by arrows 24, 26. The magnetic disk12 includes the data track 14. A strong magnet 28 is coupled to thedisk. The south pole 28A and the north pole 28B of the strong magnet 28are connected by yoke 30. The strong magnet 28 creates the detractingforce needed to create the distance between slider 2 and disk 12. Thedirection of the magnetic force is indicated by arrows 32, 34. Thevertical force (arrow 36) between the slider magnet 20 and the magnet 28of the magnetic disk 12 counteract the force of the suspension 6 (cf.arrow 38) to create a dedicated distance therebetween.

Also shown in FIG. 2 are weak magnets 40, 42 and strong magnets 44, 46on the surface of the magnetic disk 12. The weak magnets create theforces that keep the slider 2 aligned with the data track 14. Thecombination of these magnets, which applies an attractive and detractiveforce to the slider 2, creates a horizontal force to avoid displacementof the slider 2 from its track. If this functionality is not needed, themagnetic components 40, 42, 44 and 46 may be removed.

Inside the slider 2 and the magnetic disk 12, the magnetic force isguided by the yokes 22 and 30. Shields in the sensor element and thedata track to shield the magnetic fields of the yokes 22, 30 are notshown in FIG. 2.

It is also possible to exchange the magnetic structure in the slider 2with the magnetic structure in the magnetic disk 12. In other words, thestack in the magnetic disk 12 can be placed in the slider while themagnet in the slider is placed in the magnetic disk. To improve theflying and tracking performance more complicated magnetic stack designsmay be used.

The detracting force created by the levitation magnets 28A, 28B, 40, 44,42, 46 can be calculated from the total magnetic energy by using themethod of virtual displacement. Where the gap between the slider and themagnetic disk is smaller than the lateral dimension of the magnets, themagnetic energy density is equal to 0.5*H*B, with H the magnetic fieldstrength and B the magnetic induction. From this, the magnetic pressure(p) can be calculated from the volume between the levitation magnets,the surface area of, and the distance between the levitation magnets asbeing p=4*B², with B measured in tesla and p measured in bar. This meansthat, e.g., to get a pressure of 1 bar a magnetic induction of 0.5 Teslais needed.

FIG. 3 is a graph showing the force vs. the gap height, i.e., thedistance between the slider 2 and the magnetic disk 12. FIG. 3illustrates, by way of example, the principle of the inventive system,especially, how the different sources of force (spring/magnet) worktogether. The suspension creates a force towards the disk surface(attracting force—curve A). The magnets create a force in the oppositedirection (detracting force—curve B). The resulting force (curve C) iszero at a distance of 0.5 μm. At this point the effective springconstant (curve D) is 10 times higher than the spring constant of thesuspension (cf. curve A). That means that the resonance frequency of thewhole system, i.e., the slider-suspension system, is 3 times higher thanthe resonance frequency of the suspension. This, in turn, means that thesystem is very robust against vibrations or shocks.

FIGS. 4A and 4B show one design of the levitation magnets 16, 18 ofFIG. 1. The disk substrate 54, being made of non magnetic material, iscovered by a magnetic material 56, forming one disk levitation magnet18. The arrow 58 indicates the magnetic orientation. Flying above thecovered disk substrate is a slider levitation magnet 16. In theembodiment shown, the slider levitation magnet is formed by a multilayerstructure 62, but can be formed of a single layer as well. FIG. 4B showsa magnification of the slider levitation multilayer structure 62, beingformed by a sequence of non magnetic layers 64 and magnetic layers 66.The arrow 68 indicates the magnetic orientation.

FIG. 5 is a graph showing an illustrative calculation of the forcesacting on the slider 2 as a function of the flight height, using thedesign of FIG. 4.

In use, the magnetic medium 12 is coupled to an external device (notshown), e.g., a motor, spindle, gear, or the like, the rotation of whichis to be evaluated. The magnetic medium 12 can be generally planar (asdescribed), or can be formed along the outside of a generallycylindrical-shaped substrate. As the magnetic medium 12 rotates with theexternal device, data written in the data track 14 causes a measurablechange in an electrical current passing through the sensor 8, which canbe used to calculate rotational velocity. Where the position of the dataon the track 14 relative to the magnetic medium 12 is known, positionalas well as velocity data is determinable. And because of the high datadensity per track 14 that can be read with GMR and TMR sensors, veryprecise positional and rotational information can be obtained.

A variation on the above would implement an optical sensor which readsoptically recognizable markers on a surface replacing the magnetic disk.

Another variation would have the sensor 8 move with relation to astationary medium 12. Movement of the slider 2 to which the sensor 8 isattached may be difficult due to the evaluation unit and the respectiveleads attached to the slider 2. However, in case the respectiveconnections are realized without actual physical leads, e.g., wireless,or with brushes and contact tracks, this can be done very easily.

Illustrative devices that can implement the system include magneticstorage systems, FA equipment, OA equipment, etc.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of a preferred embodiment shouldnot be limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

1. A non-contact sensor system, comprising: a substrate having at leastone sensor element, the at least one sensor element being directedtowards at least one data track on a medium positioned opposite the atleast one sensor element, wherein the substrate and the medium eachcarry at least one magnetic track, wherein orientations ofmagnetizations of the magnetic tracks are such that the substrateexperiences a force away from the medium.
 2. The sensor system accordingto claim 1, wherein the at least one sensor element is selected from thegroup consisting of a magnetic sensor, an electrical sensor, a forcesensor, and a Hall sensor.
 3. The sensor system according to claim 1,wherein the at least one sensor element is a GMR sensor or a TMR sensor.4. The sensor system according to claim 1, wherein the at least onesensor element is part of a read/write head.
 5. The sensor systemaccording to claim 1, wherein the substrate is a slider.
 6. The sensorsystem according to. claim 1, wherein the at least one sensor element isdirected towards the at least one data track by a suspension.
 7. Thesensor system according to claim 1, wherein the at least one sensorelement is directed towards the at least one data track by at least oneof a spring, a magnet, and a pneumatic/hydraulic pressure element forbiasing the substrate towards the medium.
 8. The sensor system accordingto claim 1, wherein the medium is a magnetic disk.
 9. The sensor systemaccording to claim 1, wherein the at least one magnetic track include atleast two levitation magnets.
 10. The sensor system according to claim9, wherein at least one of the magnetic tracks further includes a pairof magnets flanking the levitation magnet of the magnetic track, thepair of magnets creating a force for helping maintain an alignment ofthe at least one sensor element with the at least one data track. 11.The sensor system according to claim 9, wherein at least one of thelevitation magnets is a laminate of magnetic and non magnetic layers.12. The sensor system according to claim 1, wherein the substrate andthe medium are able to perform a relative movement.
 13. The sensorsystem according to claim 1, wherein the sensor element is an opticalsensor.
 14. The sensor system according to claim 1, wherein the at leastone sensor element is, in the alternative, at least one tool adapted tomodify a surface of the medium.